Our over-all objective is to understand how the structural specialization at synaptic junctions are related to mechanisms of communication between excitable cells. Two distinct types of junctions, chemically transmitting synapses and electrotonic synapses, will be studied in order to further elucidate the morphological basis of neuro- neuronal and neuromuscular transmission. For the current year we have concentrated our research on the chemically transmitting synapses of the frog neuromuscular junction and on the Mauthner fiber - giant fiber junctions of the hatchetfish. In addition, we have studied the electron microscopy and permeability properties of a rectifying electrotonic synpase in the crayfish. In the developing axial musculature of the axolotl, transient gap junctions occur between differentiating skeletal muscle cells. Bibliographic references: Schwartz, I.R., Pappas, G.D. and M.V.L. Bennett, 1975. The fine structure of electrocytes in weakly electric teleosts. J. Neurocytol. 4:87 - 114.